This invention is related to seismic particularly to a method for obtaining seismic data having improved resolution.
Marine surface seismic surveys are traditionally obtained by towing a streamer cable up to several kilometers in length, in a series of parallel, spaced apart lines, and actuating an acoustic source at periodic intervals. Both the acoustic source and streamer cable are towed by a vessel containing appropriate navigation timing and recording equipment necessary to collect seismic data. The streamer cable extending behind the vessel contains hundreds and perhaps thousands of hydrophones spaced along its length for receiving transient seismic signals. The hydrophonss convert the received seismic signal into an electrical or optical signal which is transmitted to the recording equipment aboard the vessel. The data collected and recorded provide an approximation of the subsurface of the earth between the source and the sensors each time the source is actuated. The data collected by the hydrophones are often corrected for deviations caused by ocean currents or wind currents which cause the cable to snake or drift away from the line intended to be surveyed.
The technique briefly described above provides a series of two-dimensional representations of the subsurface. It was realized that by collecting data in one direction and then collecting data in a perpendicular direction, the resulting data could provide essentially a three-dimensional representation of the subsurface. This technique was shortened by first using two ships streaming parallel to each other which provided three linear surveys. This was later improved upon by using only one ship which was capable of deploying several streamers that simultaneously detected the reflected seismic signals.
In another attempt to increase areal coverage, at least one streamer cable is towed behind a vessel which steers circular tracks around surveyed points within the area of interest. Assuming that the subsurface reflectors are horizontal, the sweeping streamer following the vessel samples a swath of the subsurface. That is to say that the mid-point between the source and each receiver in the streamer cable tracks a slightly different concentric circle or arc. The width of the swath is determined by the radius the vessel steers and the length of the streamer cable. The locations of the hydrophones along the streamer length are determined by a complex combination of devices.
Traditionally, before or after a seismic survey of an area, it is desirable to determine the velocities of the subsurface intervals within the region of interest. This information is typically attained by conducting a vertical seismic profile (VSP) of the subsurface. Often a VSP survey is previously conducted on a borehole in or near the seismic survey. The velocities provided by that survey are used in processing the data collected in the seismic survey. If a VSP cannot be done, other techniques are available to derive the subsurface velocities, but these other methods are mere approximations in contrast to available VSP data.
As previously mentioned, VSP surveys are typically conducted before or after a seismic survey of the area. A traditional VSP survey consists of deploying one or more sensors towards the bottom of the borehole. A source for generating seismic signals is located at the surface laterally offset from the well. Two methods are often used: the first method calls for the seismic source to generate signals at a single location while the sensors are raised in incremental distances. The second method requires the sensors to remain stationary and the seismic source is incrementally moved towards, or away from the top of the well after each signal is generated.
VSP surveys provide a much better determination of the subsurface velocities because the seismic signals suffers less attenuation due to its reduced propagation distance. Additionally, the seismic signal only needs to pass through the weathered layer only once. The weathered layer often alters the signals of surface seismic data. A great disadvantage to collecting VSP data using either of the methods described above is that many variables exist which alter the detected signals, thus the determined velocities may not be as accurate as they possibly could be. One of the characteristics believed to influence the accuracy of VSP surveys is the angle of incidence of the acoustic signal upon the target reflector. It is probable that as the angle of incidence is changed for a seismic signal, the amplitude is also effected. If the amplitude is affected so might also the frequency or velocity of the signal.
The accuracy of VSP data is critical when trying to define the areal limits of petroleum deposits. The limits of a petroleum reservoir are often determined using seismic data. The VSP data collected from the borehole are correlated to the surface seismic data in order to recognize the seismic response of the reservoir, however since the seismic data and VSP data are often collected using different sources at different times, and most likely with different sensors and angles of incidence, the VSP data may not be correlatable to the surface seismic data. Thus, the reservoir limits may not always be accurately defined because of the different characteristics under which the two data sets (VSP and surface seismic) are collected.